Mechanical filter



March 19, 1940- R. HELLMANN ET AL 2,194,165

MECHANICAL FILTER Filed June 24, 1938 -l l l INVENTORS REM/HARD HELLMANN, 0770 HE YMANN ZRANZ TUCZEK ATTORNEY.

Patented Mar. 19, 1940 UNITED STATES PATENT OFF-ICE MECHANICAL FILTER Reinhard Hellmann, Jackson Heights, N. Y., and

Otto Heymann and Franz Tuczek, Berlin- Siemensstadt, Germany, assignors to Siemens & Halske, "Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Applicationfilune'24, 1938, Serial No. 215,564

In Germany June 25, 1937 '6 Claims; (01'. 178-44) Coupled mechanical oscillation systems may advantageously be used as electro-mechanical bandpass filters especially inside the tone-frequency or AF range. The closeness or strength of the mechanical coupling, among other factors, is denumber of such organizations have-been disclosed and suggested.

However, in all of these arrangements the conversion (transduction) of electrical into mechanical energy, and vice versa, is attended with dimculties in the attempt toinsureja n acceptable efficiency. In electromagnetic and electrodynamic converters there is required a considerable expenditure of material in order to preclude the channel or, transmission-range attenuation from assuming very high values. Thisdisadvantage is decisive particularly where low frequencies are dealt with. Where the frequency is high, growth of losses as a general rule affect the transmission range attenuation as well as the slope of the sides of the filter in an unfavorable sense. In other converters or transducers known in the art, for instance, those predicated upon piezo-electric and magnetrostriction phenomena, the attainable efiiciency essentially is a function of the electric adaptation or matching. The latter is never even fairly perfect, and it constitutes really the chief problem. Now, all of the drawbacks and difiiculties of known arrangements are obviated by the invention.

According to the invention a mechanical filter with an electromechanical converter is so designed that a reinforced conversion of energy is'assured. Such energy conversion is obtained by the aid of transducers with an electric or a mechanical source of energy supply, the source of energy to deliver preferably rectified energy.

Amplifying converters or transducers of this kind which comprise an electric energy supply sourcesuited for the purpose are, among others, the pressure or velocity type of transmitters (pickups) known in the prior art, and more particularly also contact-type microphones, carbonpressure regulators and other real (active) resistances variable by virtue of mechanical shifts .or displacements such as liquids with a resistance being a function of the pressure; moreover, amplifier tubes comprising a mechanically controlled grid are also suited for the above purposes.

Particularly suited has proved to be a bolometer arrangement.

, Amplifiers which contain a mechanical energy supply source could be realized, for instance, by rotary driven masses whose angular speed is subject to a control action exercised by mechanical 10 means such as friction, the like. I

The invention shall be explained in more detail in what follows by reference to the drawing wherein Fig. 1 discloses a diagrammatic perspective view of a mechanical filter according to the invention, Fig. 2 discloses a front View partly in section of a portion of the filter shown in Fig. 1, showing means for providing artificial cooling of eddy-current braking, or

the vibratory elements of the filter, Fig. 3 is a modification of Fig. '2, and Figs. 4 and 5 are circuit arrangements of the mechanical filter according to the invention utilizing feed back. In these organizations the mechanical filter is realised and embodied in the form of a tuning fork, while the reinforcing transducer or converter consists of a bolometer organization. It is assumed to be known from the prior art' that tuning-fork filters having a frequency-response curve of attenuation are designed in a way resembling that of an electric band-pass filter.

In its fundamental operation the arrangement is illustrated in and will be explained in connection with Fig. 1. A tuning fork I is excited in the usual way by the aid of a coil 2 (input of filter). Attached to and supported by the tines 3 thereof are slender wires 4 and 5, and these are included in the two arms of a bolometer bridge arrangement The latter'is so fed across two diagonal points from the source of D. C. voltage supply 6 that the wires, 4, 5, attain a high temperature. If the resistance of these wires undergoes a change, say, 'as a result of the chilling effect produced incidental to the vibration of the tines 3, an alternating voltage will arise across the'bridge resistance 1 (output'of filter). Even natural cooling such as set up as a result of the vibratory motion results in such a bridge potentialj however, inasmuch as it would lead to frequency doubling, which is undesirable, the regulation of the wire temperature should preferably be insured in an arrangement as illustratively shown in Fig. 2.

In this case'a current of air 8 produced, for instance; by a piston or diaphragm type of pump 9 is so passed through slots .lll that the heater wires 4, 5, are caused to oscillate inside a nonhomogeneous velocity field of its marginal zone. In the assumption that the temperature variation of the wires is proportional to the amount of heat carried away by virtue of convection each unit of time, there must be made this additional demand, with a view to insuring the best possible sinuous shape for the output potential, that the drop of,

speed in the marginal zone should be linear. As a matter of fact, inside a definite range this demand is always fulfilled with adequate approximation. This range governs the greatest amplitude with which the tuning fork'may be operated without the output potential evidencing nonlinear distortions.

Another exemplified embodiment is outlined in Fig. 3 where the heater wires 4, 5, are immersed in an air current which penetrates the slots H. These may be covered by vanes l2 whose active edges l3 do not extend parallel to the slots H, and which are rigidly united with the tines 3 of the fork. In this manner the effective length of the cooling air current is influenced in direct proportion to the deflection of the ends of the tines or prongs so that no special rules or directions need to be followed regarding the speed distribution in the air current at right angles to the axial direction of the wires. This arrangement as well as the one shown in Fig. 2 could, of course, be improved upon by the use of several air ports and heater wires, and these could be connected electrically either in series or in parallel relation.

The distribution of the temperature throughout the cross-sectional area of the heater wires is similar to that of the current distribution in a circular conductor which is located inside an electromagnetic A. C. field. The depth of penetration becomes less in both instances as the frequency grows. In the case of the bolometer this leads to a decline in the rate of resistance change, and thus to a diminution (and rotation) of the vector of the output potential, if the amplitude of the tines is assumed to be constant. This is immaterial because in the case of tuningfork filters generally only narrow frequency bands are involved and a constant electro-mechanical transmission ratio is desired. This fact, on the contrary, fixes the frequency limit up to which arrangements of this natiue may still be profitably utilized. The same may be extended by using in lieu of the circular heater wires, wires with a cross-sectional form in which the relation between surface area and content or volume is above such as slender ribbons or tapes,.for instance.

An essential improvement, the use of which is not restricted to the present instance, is obtained by feedback relationship between the amplifying converter device and the input of the mechanical filter, said feedback being mechanical, optical or electrical. Thus, for instance, part of the amplified output potential could be re-applied to the input end. This is accomplishable most simply in an organization of the kind shown in Fig. 4. The closeness of the coupling relation is so chosen that the product lAl-[F] stays below unity (where A amplification or gain of the arrangement, and F amount of feedback). In other words, so that no self-oscillation will be able to arise in the system. But the de-attenuating or regenerative effect of the feedback upon the filter characteristic is moreover a function of the phase of the regenerated input potential. Particularly Where special cases.

high frequencies are dealt with, the phase shift between the vibration of the tines and the output potential may be decisive for the phase balance. For this reason, the insertion of a network M in the feedback path may be requisite, the said network having the object to establish and insure such phase relation as may be desired in some If a maximum amount of regeneration is required for a single frequency, then the input potential and the feedback potential must be in phase. But if, for example, in the case of a band-pass filter the point where an in-phase-condition prevails is chosen in the neighborhood of a corner (marginal) frequency, the width of the band-pass width may thus be enlarged.

Filters which are furnished with a source of energy resembling the one just described, are dissymmetric four-terminal networks. In the use of such four-terminal networks (or quadripoles) it is often desirable that the attenuation in one direction should be infinitely high. This, indeed, is true of the described organization in the direction from the output to the input ends, provided that no feedback is used. In case of deattenuation or regeneration by means of feedback, return to the input end may be precluded, for instance, by the aid of a difierential transformer I 5 as shown in Fig. 5. In order that the flux in the transformer [5 due to the feedback current may be zero, a simulation ll; is required. Because of the fact that regeneration is thus made to include both mechanical oscillation systems, the shape of the filter attenuation curve becomes symmetric with relation to the center of the channel or transmission band. In such a scheme the point where an in-phase condition prevails between the input potential and the feedback potential may be connected for both tines or prongs and the same may most suitably be chosen each in the neighborhood of the corner frequency.

If the filter is to be symmetrical, in other words, if the input and the output ends are to be transposable or interchangeable then two like tuning fork sets could be connected together in the manner of a two-wire amplifier or repeater. auxiliary means mentioned in the above exemplified embodiment, such as D. C. SOLllCB of potential supply and diaphragm pump, need not be duplicated.

What we claim is:

1. An electro-mechanical filter comprising input terminals and a coil connected thereto. vibrating means actuated by said input coil, a bolometer bridge arrangement disposed in a heat conducting medium and cooperating with said vibrating means, a source of electrical energy connected to one pair of conjugate bridge terminals, and output terminals connected to the other pair of conjugate bridge terminals.

2. An electro-mechanical filter for low frequencies comprising input terminals and a coil connected thereto, a tuning fork actuated by said input coil, a bolometer bridge arrangement disposed in a heat conducting medium and cooperating with the tines of said tuning fork, a source of electrical energy connected to one pair of conjugate bridge terminals, and output terminals connected to the other pair of conjugate bridge terminals.

3. An electro-mechanical filter for low frequencies comprising input terminals and a coil connected thereto, a tuning fork actuated by said input coil, 2. bolometer bridge arrangement including a pair of heatable resistance wires which are disposed in a heat conducting medium and which But the Cir cooperate with the tines of said tuning fork, a

source of electrical energy connected to one pair ing the heat conducting medium which is con trolled by the filter.

6.An electro-mechanlcal filter comprising input terminals and a coil connected thereto, a tuning fork actuated by said input coil, a heatable filament disposed in a heat conducting medium mounted on each tine of said fork, a pair of resistances electrically connected to said filaments and forming therewith a bridge, a source of eleci trical energy connected to'one pair of conjugate bridge terminals, and outputterminals connected to the other pair of conjugate bridge terminals REINHARD HEILMANN. o'rro- HEYMANNLY FRANZ TUCZEK. 

